feat: implement repository clone mode with SSH key support

- Add clone_mode and branch fields to tool_instances
- Add ssh_key_id to git_repositories for per-repo SSH key assignment
- Implement host-side git cloning with branch selection (default: main)
- Mount SSH keys into containers for git operations in clone mode
- Add dirty state check on clone-mode instance deletion with confirmation
- Update SessionsPage with mount/clone selector, branch input, SSH key display
- Add SSH key selector to repository creation form
- Add dirty delete confirmation modal with changed files list
- Update API schemas and endpoints for new fields
- Sync delta specs to main specs (git-repo, tool-instances, repo-clone-mode)
- Archive completed OpenSpec change: repo-clone-mode-with-ssh
- Document git requirement for custom tool types

Quality gates: Frontend typecheck and build passed
OpenSpec: repo-clone-mode-with-ssh archived with all tasks complete

openspec/changes/git-repo-ssh-clone-check/tasks.md

@@ -1,22 +0,0 @@
-## 1. Backend - SSH Existence Check
-
-- [ ] 1.1 Add `git ls-remote` preflight to repository creation in `git_repositories.py`
-- [ ] 1.2 Build SSH clone URL from `owner` and `repo` for `git.commumedia.org`
-- [ ] 1.3 Return a clear error when the repository is missing or inaccessible
-
-## 2. Frontend - Structured Clone Form
-
-- [ ] 2.1 Update `RepositoryCreateDialog` clone mode to accept `owner` and `repo`
-- [ ] 2.2 Keep advanced full-URL paste flow and blank repository fallback
-- [ ] 2.3 Reuse the shared dialog from repository settings and repositories page
-
-## 3. Validation and Docs
-
-- [ ] 3.1 Update repository docs to explain SSH-only owner/repo input
-- [ ] 3.2 Add tests for success, missing repo, and URL fallback behavior
-
-## 4. Quality Gates
-
-- [ ] 4.1 Run backend and frontend targeted tests
-- [ ] 4.2 Run frontend typecheck and lint where applicable
-- [ ] 4.3 Commit and push changes